The invention relates to a process for manufacturing 5-hydroxymethylfurfural, denoted by HMF in the following, of the formula: ##STR1##
HMF is a conversion product of hexoses with 6 carbon atoms.
The conversion of the sugars is done in an acid medium and under the action of heat. It is characterized by a dehydration followed by cyclisation.
The hexoses comprise aldohexoses which have an aldehyde function and ketohexoses which have a ketone function.
Among aldohexoses, may be mentioned, for example, glucose, galactose, mannose, iodose and, among ketohexoses, fructose or levulose, sorbose, tagatose and allose.
More generally, any oligo- or polysaccharide whose transformation leads to aldohexoses and/or ketohexoses can be used as a starting material for the invention.
The mechanism of formation of HMF is represented generally by the diagram indicated below. Under the effect of acid on hexoses, intermediate products are formed whose structure is still very poorly understood and which give rise either to HMF, or to a group of compounds, called humins, which are insoluble polymerisation products. According to the operational conditions, the development of the intermediates will be oriented towards the formation of HMF or towards that of humins.
HMF can itself result in by-products either by opening of the ring (levulinic acid and formic acid), or by polymerisation thereby providing humins. ##STR2##
Applicants have already developed a process for manufacturing HMF by a conversion of hexoses, this process consisting of bringing the hexoses in aqueous solution in contact with an exchange resin with cationic function at a temperature below 100.degree. C. and recovering the HMF formed by means of a solvent for the latter selected from among those whose water miscibility is as low as possible.
This process gives satisfaction, but in view of more and more severe economic requirements, Applicants have sought to improve it in the direction of an increased yield.